The present invention relates to an intermediate frequency amplifier circuit and, particularly, to an amplifier circuit which receives an intermediate frequency output through a crystal filter used for a satellite communication facility.
It is known in a conventional intermediate frequency amplifier circuit that, as shown in FIG. 1, a resistor network circuit 4 is inserted in between a crystal filter 1 and an amplifier 5 to perform an impedance matching therebetween. In this case, an amplifying element in the amplifier 5 is a transistor 7. Since an output impedance of the crystal filter 1 is in the order of 75 .OMEGA. while an input impedance of the amplifier 5 is in the order of several k.OMEGA., band characteristics of the intermediate frequency amplifier circuit will be degraded due to impedance mismatching if they are connected directly. Therefore, such impedance matching by means of the resistor network circuit is necessary. In order to make the impedance matching in the intermediate frequency amplifier circuit as reliable as possible, the resistor network circuit is usually constituted with a .pi. type attenuator.
In such a conventional intermediate frequency amplifier circuit, a signal from the crystal filter is attenuated by the attenuator 4 and thus a gain of the whole intermediate frequency amplification is lowered. Further, the noise figure (NF) is degraded by thermal noise generated by the resistance thereof, resulting in degradation of the SN ratio.